1. Field of the Invention
The present invention is directed to a method for operating a magnetic resonance apparatus having a gradient tube at which at least one gradient coil, in which current flows during operation, is arranged, and at which a number of elements are arranged for generating a force as needed that acts on the gradient tube to counteract oscillations of the tube.
2. Description of the Prior Art and Related Application
With a magnetic resonance apparatus, tomograms of an examination subject, usually a patient, can be obtained through specific body planes. This occurs with the use of electromagnetic fields. In order to enable spatial resolution of the signal obtained in the presence of an applied magnetostatic basic field and an exciting radio frequency field, a gradient field is produced with a number of gradient coils. Gradually, three different gradient coils are utilized that produce fields in the x, y, z directions with respect to the gradient tube. Due to the flow of current in these coils, Lorentz forces occur that act on the gradient tube and cause it to oscillate due to their time curve. These mechanical oscillations in turn cause the air around the gradient tube to exhibit fluctuations in air pressure. These oscillations are the cause for a considerable development of noise during the operation of the magnetic resonance apparatus. Noise peaks far above 100 dB occur. In order to oppose these oscillations and, consequently, to dampen the noise, it is known, for example from German OS 44 32 747, to generate opposing forces with piezoelectric elements that are arranged at the gradient tube and to thus cancel the oscillations excited by Lorentz forces. The arrangement of the piezoelectric elements disclosed therein, however, ensues essentially in the region of the coil conductors. The described arrangement is non-selective in view of the actually generated oscillations; a targeted noise damping is consequently not possible.
In order to achieve noticeably improved noise damping, German Patent Application 198 29 296 corresponding to pending U.S. application Ser. No. 09/343,848, filed Jun. 30, 1999 (xe2x80x9cMagnetic Resonance Apparatus,xe2x80x9d Dietz et al.), discloses exciting one or more natural oscillation modes of the gradient tube with the elements arranged at the gradient tube, while opposing the oscillations of the gradient tube produced by the Lorentz forces. It has been shown that each oscillation of the gradient tube is a superimposition of a number of natural oscillation modes, i.e. each oscillation can be reduced to specific natural oscillation modes. The natural oscillation modes can supply different contributions to the actual tube oscillation; the elements, however, allow specific natural oscillation modes to be intentionally and specifically excited, so as to oppose the respective natural oscillation mode components of the tube oscillations, and thereby eliminating them. A considerable reduction of the noise can be achieved as a result. Change in the oscillatory behavior of the gradient tube can occur, however, during the operation of the magnetic resonance apparatus or during a longer operating time thereof. These can be reversible or irreversible modifications of the initial conditions. A rigid drive spectrum of the elements, i.e. a force on the gradient tube generated by the elements that is always constant, is non-specific in view of the changes which may occur, and can no longer adequately compensate these.
It is an object of the present invention to provide a method of the type initially described that enables compensation of vibratory changes as may occur during the operation of a magnetic resonance apparatus.
This object is inventively achieved in a method for operating a magnetic resonance apparatus having a gradient tube at which at least one gradient coil is mounted that has current-flowing therein during operation, and at which a number of elements are attached for generating a force as needed that acts on the gradient tube to counteract oscillations (vibrations), wherein one or more natural oscillation modes of the gradient tube are excited with the elements, to oppose the oscillations of the gradient tube produced by Lorentz forces that are generated as a consequence of the flow of current through the gradient coils, and wherein frequency-dependent drive signals are used for the excitation of the elements, the amplitudes and/or the phases of the drive signals for modifying the force generated by the elements and acting on the gradient tube having varied for compensation of a change of the oscillatory behavior of the gradient tube, dependent on at least one measured value representing a criterion for the change of the oscillatory behavior.
In the inventive method, at least one measured value is identified, which serves as a criterion for the change of the oscillatory behavior, i.e. the modified oscillatory behavior is directly or indirectly acquired with the measured value. Dependent on this measured value, a modification of the drive signals of the force-generating elements subsequently ensues, i.e. the generated force is varied and set dependent on the oscillatory change, so that this oscillatory change can be largely compensated. The variation ensues by modifying the amplitude and/or the phase of the force, these being available as variable quantities, but it is primarily variation of the amplitude that is suitable for compensation. The phase of the drive is usually very stable, since the force of the force-generating elements must always oppose the Lorentz forces and this is only possible in a well-defined phase relationship; due to a time-delayed response behavior of the force-generating elements relative to the control signal, nevertheless some potential oscillatory changes resulting therefrom can be compensated by phase variation. Changes in the oscillatory behavior of the tube can be determined directly from the gradient tube. For example, as a result of the flow of current through the gradient coils, the tube may be heated. This change is reversible, i.e. the oscillatory behavior also changes correspondingly when the tube cools. In addition, changes can be produced, for example, by aging of the tube, for example, resulting in a change of the modulus of elasticity, caused, for example, by the cyclical heating and cooling that repeatedly occur during prolonged use. The attenuation of the gradient tube also may change. Further, the influence of the force of the force-generating elements on the tube can change, for example due to fatigue of the material with local modification of the material properties; the force-generating elements themselves also can age, so that the force that is generated decreases despite the same drive. The modifications of the oscillatory behavior resulting in one or more of these sources can be advantageously compensated with the inventive method.
When the modification can be attributed exclusively to a change in a force-generating element (for example, partial or complete failure), then this change should likewise be compensated at only this element. (Replacement or adaptation of the drive of this one element to the required force) Respective sensors can be disposed at each force-generating element which, when the force-generating element is being driven, serve as a force-measuring sensor whose signal is used only for the controlling drive of the one element at which it is desired. When the force-generating element is not being driven, the sensor measures excursions generated by the other force-generating elements relative to a previously identified reference value. An arbitrary sensor (excursion, acceleration, expansion) can be used for this purpose.
Further features relate to all force-generating elements of a group in common:
It has proven expedient when the change of the drive signals ensues on the basis of a change of at least one drive curve stored in a control unit, the drive curve representing frequency dependent values for the force to be exerted by the elements on the gradient tube so as to generate the natural oscillations. Such drive curves exist for each group of elements that are provided for exciting a specific natural oscillation mode and represent a frequency characteristic that indicates the amplitude and phase for the signal to be provided to the force-generating elements of an element group in order to compensate the effect of the Lorentz force of a sinusoidal excitation having a normalized strength of one at the respective frequency. The effect must be set by means of the phase so that the influence of the force of the force-generating elements opposes that of the Lorentz force. A different, higher or lower force is to be exerted due to the change in oscillation, this being able to be easily taken into consideration by variation of the xe2x80x9cforce drive curvexe2x80x9d, based on which the actual drive signals are calculated in the control unit. The determination of the drive signals can inventively ensue by separating the individual, direction-dependent signal curves for the respective gradient axes from a time-dependent drive signal curve for the gradient coils located at the gradient tube, and frequency-dependent drive signal curves are generated by Fourier transformation. These curves as subsequently superimposed with the drive curve or curves that are respectively allocated to a specific group of elements of the respective gradient axis, after which the superimposition curves that are obtained are converted by Fourier back-transformation into specific time-dependent drive signal curves for the respective element groups. This manner of determining the drive signals is advantageous from two points of view. First, it makes it possible to react to the time signal of the gradient current, which is extremely important insofar as the gradient current is the cause generating the Lorentz forces. By computational operation or by taking the gradient current signal into consideration in the framework of the determination of the drive signals for the elements, the time component of the gradient current can be taken into consideration within the time-dependent drive signals for the force-generating elements. The second advantage of this embodiment of the method is that since the drive signal curve always remains the same for the gradient coils, only the force drive curves need to be varied in the framework of the determination, i.e. only one processing parameter, namely the force drive curve to be superimposed, is modified within the computational procedure in order to obtain the required, new drive characteristics serving the purpose of compensation.
A measured value for a reversible oscillation change can be inventively identified, for example the temperature of the gradient tube. Even though compensation can be achieved by a suitable variation of the element group-specific drive curves originally stored, it has proven expedient to store a family of drive curves in the control unit. Each family can be allocated to a specific measured value or to a measured value interval, and a drive curve on which the drive signal is to be based can be selected therefrom dependent on the measured value.
Alternatively or additionally, the oscillations of the gradient tube can be registered as further measured values related to natural oscillation. This applies both for compensation of oscillatory changes caused by temperature fluctuation as well as for determining system changes that are quasi non-reversible and, for example, are attributable to fatigue phenomena, material changes or power losses in the force-generating elements. The amplitudes of the intrinsic oscillations can be measured as the measured values. It has proven especially expedient when the drive signals are inventively varied for compensation with substantially simultaneous identification of the measured values related to natural oscillation and the effect of the variation is checked on the basis of the measured values. In this case, a control circuit is employed wherein the compensation effect is checked during or after a modification of the drive signals. When, for example, the force exerted by an element group for generating a specific natural oscillation mode is increased for compensation, it can thus be recognized after the first or after a few incremental steps whether an incrementation in fact leads to a compensation. If not, variation by means of the control unit can ensue such that the generated force is reduced, the effect likewise always being checked in this case as well. The modification of, for example, the amplitude of the drive signals or of the drive curve as the cause thereof, thereby occurs until a compensation optimum has been achieved. It has proven adequate when only one frequency-related signal of the drive curve of the elements is inventively varied with respect to each intrinsic oscillation to be compensated. Alternatively, it is also possible to vary a signal group of a specific frequency range.
A number of sensor elements arranged at the gradient tube can be inventively employed for determining the measured values. The sensor elements for determining the temperature should be arranged close to the elements themselves since the temperature of the force-generating elements, which can be responsible for a change in the function thereof, can thereby also be simultaneously acquired. For determining the oscillations of the gradient tube, further, at least as many sensor elements as there are natural oscillation modes to be suppressed should be employed. The different oscillation modes are known; the number of important modes is surveyable. In the ideal case, just as many sensor elements thus exist as there are oscillatory modes per spatial direction to be taken into consideration. Mathematically considered, the natural oscillation modes form a vector space and the sensor elements represent the supporting points at which the function is known. The placement of the sensor elements must be such that an adequate input signal that is forwarded to the drive unit is made available. The sensor elements thus should not lie on a common node line of a number of modes or assume symmetrical positions that do not deliver any relevant information. The placement of the sensors within a small segment region of the tube is sufficient i.e. they need not be distributed over the entire tube.
In addition to being directed to the method, the invention is also directed to a magnetic resonance apparatus having a gradient tube at which at least one gradient coil is mounted that has current-flowing thereon during operation and at which a number of elements are arranged for generating a force as needed that acts on the gradient tube, with the position of the elements being selected dependent on at least one natural oscillation mode of the gradient tube, so that the respective natural oscillation mode can be excited during operation of the elements, and wherein at least one sensor element is provided for determining at least one measured value that represents a criterion for a change of the oscillatory behavior of the gradient pipe, and wherein a control unit varies the amplitude and/or of the phase of frequency-related drive signals, with which the elements are driven, dependent on the measured value, so that the change of the oscillatory behavior can be at least partially compensated.
Force sensors as well as temperature sensors can be employed as the sensor elements, as can sensor elements that supply natural oscillation-related measured values for the tube oscillation itself. At least as many oscillation sensors as there are natural oscillation modes to be suppressed can be inventively provided. The sensor elements should be arranged in the longitudinal tube direction and in the circumferential tube direction and should be spaced substantially equidistantly from one another. It is sufficient for the sensor elements to be arranged only over a partial length and a partial circumference of the gradient tube, for example, a xe2x85x9 segment. For example, acceleration sensors, expansion sensors or force sensors can be employed as the sensor elements.